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I am being plagued with client errors trying to send through my SMTP server (postfix).

On a fast internet connection it works no problem, no error. On a slow (satellite) connection, it constantly returns this error:

Task 'person@example.com - Sending' reported error (0x800CCC60) : 'Your outgoing (SMTP) email server has reported an internal error. If you continue to receive this message, contact your server administrator or Internet service provider (ISP). The server responded: c220 box.tuilder.com ESMTP Hi, I'm a Mail-in-a-Box (Ubuntu/Postfix; see https://mailinabox.email/)'

This is an outlook client, but mac mail clients have the same problem.

It seems that there is a timeout occurring somewhere along the line. Not sure if I can tweak a postfix setting. Have tried a few, but no progress.

Any suggestions?

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The sample /etc/postfix/master.cf file contains, among others, these lines:

relay     unix  -       -       n       -       -       smtp
#       -o smtp_helo_timeout=5 -o smtp_connect_timeout=5

You can uncomment the second line and increase the numbers. 5 seconds can indeed be too little with bad connections.

NATing routers also have connection timeouts, but these usually go in the order of minutes - unlikely to cause you problems. But check /proc/sys/net/netfilter/nf_conntrack_tcp_timeout_established just in case. Run

cat /proc/sys/net/netfilter/nf_conntrack_tcp_timeout*

and see if there is any low value that you could increase. On my machine, the only ones below 20 seconds are for closing connections (the first two), but since your SMTP session appears to have failed after HELO, that isn't the problem.

  • Un-commenting that line and changing the timeout to 15 seconds has helped a lot. – Emmanuel Jun 20 '18 at 3:02

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